Have you checked the throttle cable itself? It may sound dumb, but my idle was going crazy, high, low, fluctuating, etc. I did two things: made sure the cable was all the way out, with a little slack, and then tightentned a clear vacumn hose on the intake manifold next to the throttle cable. It now idles perfectly. If the cable has no slack, it is like constantly being on the gas. The computer begins to "hunt" for the correct air / fuel mix, and the idle is not correct. Check this first. Is your "check engine" light on? If not, I can't see why it failed emissions test. If your oxygen sensors have not been replaced in 30-60k+ miles, then they could be the problem, and will cause the emissions failure, and maybe the idle? To clear the computer, disconnect the batt for a couple of hours and try again.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro